The invention relates to a distributor head for forming a tubular profile from one or more streams of extruded thermoplastic material, which distributor head comprises at least an inlet orifice for a stream of plastics material, which can be connected to an extruder, and an outlet orifice of annular cross section.
In many cases, extruders which are connected at their output side to a distributor head are used to produce tubular profiles from thermoplastic materials. In the case of profiles having a wall consisting of a single layer, said distributor head is generally positioned in line with the extruder, so that the material stream from the extruder passes through in a virtually rectilinear manner. For certain applications and materials, distributor heads are also known in which the direction of the material stream is altered. Said distributor heads are generally referred to as "crossheads". In comparison with the aforementioned distributor heads which are positioned in line with an extruder and which are also known as "straight heads", crossheads of this type have the advantage, inter alia, that the interior of the extruded tube can be accessed easily, for example in order to fit a cooled core. When manufacturing profiles having a wall consisting of a plurality of layers, for example three layers, it is also known to position at least one of the inlet orifices at an angle to the centre line of the outlet orifice. In "straight" distributor heads, where a material stream is fed along the centre line of the outlet orifice and which have an inner core attached to the distributor head, the essentially annular stream of plastics material around the core is interrupted in at least one location by an element which connects the core to the housing of the distributor head. In the case of a distributor head in which at least one inlet orifice is positioned at an angle to the centre line of the associated outlet orifice, the material stream fed in through this inlet orifice is separated in an annular chamber into a number of branch streams and downstream thereof, said branch streams are combined to form a closed ring at the outlet orifice. In both cases, the material introduced into the distributor head at the respective inlet orifice flows to the associated outlet orifice along different paths. The disadvantage in this case is that these flow paths have a mutually different flow resistance. In addition, the fact that the length of the different flow paths may differ appreciably is disadvantageous. As a result, the residence time of the plastics material inside the distributor head varies and is difficult to determine, which has an adverse effect on the process control and process adjustment. These disadvantages occur to a considerable degree particularly if the material stream in the distributor head has to change direction.
A sensitive material such as, for example, normal rigid polyvinylchloride (PVC) can be processed using a "crosshead" only if particular attention is paid to the additives which are mixed in with the PVC in order to ensure the necessary stability. However, these additives are expensive, adversely affect the properties of the material alter extrusion and in many cases pollute the environment.